The Internet provides access to a wide variety of resources. For example, video and/or audio files, as well as web pages for particular subjects or particular news articles, are accessible over the Internet. Access to these resources presents opportunities for content items (e.g., advertisements) to be provided for presentation to a user with the resources. The pairing of content items with requested resources can generate value by increasing the likelihood that users will interact with content items. For example, a web page can include content slots in which advertisements or other content items (e.g., search result snippets) can be presented. These content slots can be defined in the web page or defined for presentation with a web page, for example, in a pop-up window.